Aortic Aneurysms (AAA) and Thoracic Aortic Aneurysms (TAA) are weakened areas in the aorta that form balloon-like bulges, or sacs, in approximately the abdominal area. As blood flows through the aorta, the pressure of the blood pushes against the weakened wall, causing it to enlarge and often rupture. Ruptured AAA is a major cause of death in the United States.
In the past, clips and open surgery were the traditional interventional treatments for AAA and TAA. More recently, endografts, sometimes with stents for added stability, have been placed across the aneurysm to reduce the pressure on the aneurysm wall and prevent its enlargement and rupture.
Most recently, systems have been described where an expandable member of a device is introduced into the aneurysmal sac by means of minimally invasive surgical (MIS) techniques, e.g., guidance through the vasculature of a human patient using a guidewire. Flowable precursor materials are introduced into the expandable member, and the precursor materials undergo a chemical reaction, thereby causing the expandable member to expand and conform to the shape of the aneurysmal sac. As the materials harden, they lock the expandable member in place in the patient and stabilize the aneurysm. See, for example, U.S. Pat. Nos. 7,872,068 and 8,044,137, and U.S. Publication No. 2006/0222596, the contents of which are hereby incorporated by reference herein in their entirety. The expandable member may be, for example, a single-walled or double-walled balloon or endobag, or an inflatable cuff. Other examples of devices having an inflatable or expandable member are provided, for example, in International Publication No. WO 00/51522, U.S. Pat. Nos. 5,334,024, 5,330,528, 6,312,462, 6,964,667, and 7,001,431, and U.S. Publication Nos. 2004/0204755 and 2006/0025853, the contents of which are hereby incorporated by reference herein in their entirety.
The flowable precursor materials are typically polymer precursors which polymerize and cross-link to form hydrogels. One preferred type of polymer precursor is a material that can be polymerized by free radical polymerization. Typically this involves the polymerization/cross-linking of two prepolymers, each having terminal reactive groups that are susceptible to free radical polymerization, such as acrylates. The polymerization is effected by combining both prepolymers with a thermally activated low temperature free radical initiator and an initiator catalyst at physiological temperature.
To avoid unintended polymerization, i.e. prior to mixing all the components and allowing them to polymerize in situ in the expandable device, the components are typically stored in two separate aqueous solutions, one solution comprising one polymer precursor and the free radical initiator, and the other solution comprising the other polymer precursor and the initiator catalyst. In this way, the two polymer precursors are kept apart, as are the free radical initiator and the initiator catalyst.
In practice, the two solutions are delivered simultaneously or nearly simultaneously and then mixed, either ex vivo in a manifold, or in the expandable device itself. Because of the instability of thermally activated low temperature free radical initiators, the solution containing the free radical initiator must necessarily be kept frozen, i.e. at zero degrees Celsius or lower, until needed. Even so, the useful shelf life of the device or kit comprising such solutions is only 12 to 18 months.
The necessity that at least one solution be kept frozen is a serious practical disadvantage, inasmuch as the solution cannot easily be thawed and be ready for use as soon as a patient presents with an AAA that needs immediate treatment, particularly since rapid thawing by conventional techniques using large temperature differentials, e.g., hot water or microwave defrosting, cannot be used because of the thermal activation of the initiator.
What is needed is a material suitable for use in forming a hydrogel and characterized by a stable composition at ambient conditions, a method of making the material, and a method of preparing a hydrogel using the material. Surprisingly, the present invention meets this and other needs.